Conventional tractor-trailer units consist of a tractor, which is connected to the semi-trailer by a hitch comprising a so-called fifth wheel member engaged with a pin connected to the trailer. The wheels of the semi-trailer unit are mounted on one or more fixed axles near the rear of the trailer. In turning a corner, the rear wheels of the trailer unit follow inside the curved path taken by the pulling tractor. This means that when a tractor drawn trailer is negotiating a sharp corner (i.e., 90 degrees), the driver must make a very wide swing with the tractor in order to insure that the trailer axles will not cut across the corner and leave the roadway.